1. Field of the Invention
This invention relates to a pneumatic tire and more specifically to the modified tread of the tire.
2. Description of the Art
Vehicles have generally become smaller and tighter, or sharper, in steering response and sensitivity. The distribution of forces and moments on the tires from the ground surface at the contact patch interface are becoming more critical for obtaining good vehicle steering stability and handling. For example, steering pull is manifested as a tendency for a vehicle to drift left or right when the steering wheel is released. This pull is caused by a twisting moment on the tire about a Z-axis normal to the contact patch, and a lateral force perpendicular to the forward velocity of the vehicle along a Y-axis at the contact patch. This Z-axis moment is called the aligning torque and the Y-axis force is called the lateral force on the contact patch.
When a midcircumferential plane of the tire has an orientation at a small slip angle (say 0.25 degrees) with respect to its forward velocity vector, the lateral forces on the contact patch can be reduced to zero. This small slip angle is called the neutral slip angle. However, there remains a Z-axis moment at the contact patch called the residual aligning torque (RAT). Also, there is another small slip angle (say 0.35 degrees) of the tire plane with respect to the forward velocity vector where the Z-axis moment is zero. At this small slip angle a Y-axis force exists called the residual cornering force (RCF). Generally speaking, it is not possible to achieve a zero aligning torque at the same slip angle that yields a zero cornering force. In fact, most vehicle/tire combinations operate at a small steady state slip angle to yield a zero lateral force, and the steering system is used to provide a torque to compensate for the residual aligning torque. When the steering wheel is released, the vehicle will drift right or left depending on the magnitude and direction of the residual aligning torque. Vehicle manufacturers have established limits on the amount of drift allowed. For example, one vehicle manufacture limits the drift from tire sources to 3 meters in a forward distance of 100 meters, or a drift angle of 1.72 degrees. The theory of tire induced steering pull is documented in the Society of Automotive Engineers (SAE) Publication No. 750406. Aligning torque and lateral force are defined as they relate to the commonly used terms of plysteer and conicity in SAE Publication No. 870423.
It is known in the art that the tire belt structure and its cross-ply laminate of reinforcing members can cause a residual aligning torque or a residual cornering force. This is due to the bending-shear coupling of the belt package and especially due to the bending-twisting coupling within the belt package when the tire contacts the ground surface. The bending-twisting deformations in the contact patch have a greater significance than the bending-shear deformations. Small shear deformations have less impact than small twisting deformations because the twisting more directly affects the local contact stresses in the contact patch. SAE Publication No. 870423 discloses the influences on residual aligning torque due to both reinforcing member spacing and reinforcing member angle from a midcircumferential plane for a belt package having two belt plies. The publication also discloses the effect of having different bottom belt ply reinforcing member angles compared with top belt ply reinforcing member angles on the residual aligning torque. No procedure to eliminate residual aligning torque by belt reinforcing member changes was disclosed in this publication.
It is also known in the art that major modifications in the belt package construction can reduce an average lateral force at a zero slip angle (plysteer) to a small value. The average lateral force at zero slip angle is determined by rolling the tire about its axis of rotation and measuring a first lateral force magnitude then reversing the rotation of the tire and measuring a second lateral force magnitude. The average of these two lateral force magnitudes is called "plysteer" in the literature. In U.S. Pat. No. 3,945,422 plysteer is substantially reduced by constructing the belt package with multiple plies (3) which are symmetrically disposed. Other multiple belt package configurations are disclosed in SAE Publication No. 760731. However, reducing the plysteer to zero does not eliminate the residual aligning torque at the zero neutral slip angle. Furthermore, these references do not teach how to make modifications in the construction of a tire to reduce residual aligning torque.
It is also known in the art that the tread pattern and tread structure also have an effect on the residual aligning torque which is independent from the construction of the tire. Tread pattern and tread structure effects can have the same relative impact on residual aligning torque as changes in the belt package construction have.
The design of the tread effects both the residual aligning torque and the residual cornering force. When the tread pattern changes, the stiffness of the tread blocks are modified. For example, changes in the circumferential grooves in the tread pattern will change lateral stiffness and effect the residual aligning torque and the residual cornering force. Changes in lateral grooves can also modify the stiffness of the tread of the tire and cause bending-twisting deformation changes within the tread. Changes in the lateral groove angles can result in less differential contact patch tangential forces between the various tread elements. Hence, the tread becomes more compliant as the tire rolls. U.S. Pat. No. 4,819,704 discloses how modifications in the size and shape of tread blocks produced by circumferential and lateral groove changes reduce plysteer. The angle of the direction of the maximum shear rigidity of the tread blocks is specified between 40 and 75 degrees from a midcircumferential plane, and is opposite to the angle of reinforcing members in the outermost belt layers. The total surface area of the tread blocks is also disclosed as a factor in reducing plysteer. However, the reference does not disclose a procedure to change the residual aligning torque, and the reference does not teach how to change lateral groove angles without influencing tread induced tire noise and traction. The importance of tire lateral groove angles on noise is disclosed in U.S. Pat. No. 5,125,444.
It is also known in the art that tread rigidity along with asymmetrical treads can effect plysteer. For example, U.S. Pat. No. 5,016,695 discloses a directional tire having an asymmetrical tread pattern wherein the rib having the highest rigidity (no lateral grooves) is positioned to one side of the midcircumferential plane. This tread pattern alters the shape of the contact patch to give excellent driving stability at slip angles as small as 1 degree, and improves ride comfort during straight ahead traveling. However, there is no discussion in this patent of the effect of this tread pattern on residual aligning torque is given, and the disclosed driving stability is at angles larger than nominal neutral slip angles where the lateral force is zero.
The tread surface profile also influences the aligning torque and lateral forces on a free rolling tire. A difference in the tread radius and differences in rates of ground contact area were disclosed to promote the maneuverability of a car in Japanese Patent No. 57-147901 (JP). In this reference, the tread radius on the outside of the midcircumferential plane is made larger than the tread radius on the inside. This difference in tread radius and difference in shape of the ground contact area (contact patch) causes a conicity force. This basic construction of the tire results in one shoulder having a 1-2 millimeter larger radius than the other shoulder. This difference does not change with the rotation of the tire, therefore, the conicity force is in the same direction with clockwise or counterclockwise rotation. Opposed conicity forces exist when such a tire is mounted on the left compared with the right side of the vehicle and with the same side of the tire mounted to the exterior.
In addition to the influence of tread pattern and tread surface profile changes on the tire's contact patch forces, tangential stresses at this interface can be changed by the inclination of the tread blocks. The angle between the tread surface and the approximately radial faces of the lateral grooves has an important effect on the traction and uneven wear performance of the tread. This angle is also important in the driving and braking forces achieved by a tread block, especially on snow, ice and rough ground surfaces. The influence on overall driving and braking pull of a tread pattern having sloping tread block elements is disclosed in Japanese Patent Nos. 63-97,405 (JP) 2-293,206 (JP), and 2-293,205 (JP). The performance of the tire on ice, snow and rough roads are enhanced by the tilting of tread blocks forward or backward.
In JP 2-293206 the tire is actually reversed from early days of wear to last days of wear to take advantage of the changing stiffness of the tread blocks with wear. The disclosure of JP 63-97,405 optionally combines tread blocks to give a tread pattern that functions effectively on respective road surfaces. However, these patents do not teach how inclined tread blocks can be positioned and sloped to have an influence on the residual aligning torque. The aligning torque on the tires disclosed would be random as optionally combined, and may in fact increase the magnitude of the residual aligning torque. Japanese Patent No. 2-293,205 discloses similar sloping tread blocks resulting from the inclination of approximately radial faces of the lateral grooves to improve drive and brake performance. No specific tread pattern is illustrated in this patent.
Other similar patents which disclose sloping tread blocks which result from the inclination of lateral grooves in circumferential ribs are U.S. Pat. Nos.: 3,104,693; 4,284,115; 4,298,046 and 5,044,414. Durability of the tire at high speed is the problem addressed in U.S. Pat. No. 5,044,414 and improved by lateral groove shape and groove bottom curvature. The same problem and a similar solution is disclosed in U.S. Pat. No. 4,284,115. Tires having improved gripping and longitudinal adherence with treads biting into the rough road surfaces as well as ice and snow surfaces are disclosed in U.S. Pat. No. 3,104,693 and U.S. Pat. No. 4,298,046. Once again, driving and braking performance of the tire as a whole is disclosed. Random residual aligning torque values are anticipated when using the treads of these patents.
A tread pattern having modified ribs based on the direction of plysteer due to tire construction and the ground contact reaction force is disclosed in U.S. Pat. No. 4,305,445. This patent describes how the wear is influenced by the direction of "internal camber thrust" acting on the tire. The ground contact pressure is modified by providing small holes near the leading edge of lateral groove surfaces on one side and small holes near the trailing edges of lateral groove surfaces on the other side of the midcircumferential plane. This modifies the rigidity of tread blocks as they enter the contact patch on one side and exit the contact patch on the opposite side. No indication is given as to influence of these small holes on the plysteer or residual aligning torque, and random influences can be anticipated.
The sloping tread blocks disclosed in U.S. Pat. application Ser. No. 07/652,412 are provided to control uneven wear on a directional tire having an asymmetrical tread pattern. The two lateral ribs, each having tread blocks sloped in the same direction, have reduced braking forces from tread block radial deformations. The central ribs each having tread blocks sloped in a reverse direction, have reduced driving forces from tread block radial deformations. The driving axle tires are reverse rotated from the steer axle tires. No changes in the residual aligning torque is anticipated from the tire treads of this invention.
Even though there are different known ways to reduce plysteer, there remains a need to be able to control and reduce the residual aligning torque on the tire from the ground surface. This residual aligning torque remains even after the tread pattern and tire construction have been modified to reduce plysteer or conicity. This moment or torque exists even at a small slip angle (neutral slip angle) or at zero slip angle when plysteer is zero. These corrections are made difficult by a desire to avoid changes in the tread pattern that influence other tire performance characteristics. The optimum solution is to reduce the residual aligning torque to approximately zero with little or no change in the tire's construction and basic tread pattern in contact with the ground surface (contact patch). Such a solution would maintain the noise, traction and wear performance of the tire. There is no need to address improvements in driving or braking traction to eliminate steering pull when the steering wheel is released. Hence, in accordance with this invention, the tire should be first optimized for noise, traction and wear by changes in the tire's construction and tread pattern, then certain modifications can be made in the tread to reduce the residual aligning torque without influencing the initial optimization.